1. Field of the Invention
The present invention relates to a mold clamping apparatus for a molding machine, for opening/closing or clamping stationary and movable metal molds in a molding machine such as a die casting machine or an injection molding machine.
2. Prior Art
A molding machine such as a die casting machine or an injection molding machine generally comprises two stationary platens having four corners connected by tie bars and a movable platen located between the two stationary platens and supported to be movable forward and backward by the tie bars. A stationary metal mold and a movable metal mold are mounted on one of the stationary platens and the movable platen, respectively. The movable metal mold, which is moved by a hydraulic cylinder directly or via a toggle mechanism and closed with respect to the stationary metal mold, is clamped by a mold clamping cylinder directly or via a toggle mechanism. Thereafter, a molten metal or a molten plastic is injected into a cavity of the metal molds and solidified, thereby manufacturing a molded product.
Such a conventional molding machine, however, has three platens, i.e., two stationary platens and a movable platen. Therefore, if a machine is of a vertical clamping type, the height of the machine is increased. If a machine is of a horizontal clamping type, the total length of the machine is increased.
The present applicant, therefore, has proposed an apparatus having only one stationary platen as disclosed in Japanese Patent Laid-Open No. 61-238456. In this apparatus, tie bars as mold clamping pistons each having a distal end portion connected to a corresponding one of a plurality of cylinders fixed to the stationary platen are inserted to be movable forward and backward in the cylinders. A movable platen is supported by the tie bars via guide bushes and connecting means such as split nuts to be threadably engaged with threaded portions of the guide bushes. A mold opening/closing cylinder is located between the stationary and movable platens. Upon mold clamping, the movable platen is moved by the mold opening/closing cylinder while the split nuts are open, thereby performing mold closing. After the split nuts are closed to connect the movable platen and the tie bars, the mold clamping cylinder is operated to perform mold clamping.
In such a conventional mold clamping machine, however, the split nuts are not smoothly threadably engaged with the threaded portions, resulting in poor operability and uncomfortable noise generated upon engagement. In addition, since the movable platen is supported by the tie bars via the guide bushes, the movable platen tends to be inclined by a gap formed in order to facilitate sliding of the movable platen. As a result, no satisfactory mold clamping precision can be maintained, and burrs are discharged.
In addition, in the conventional mold clamping apparatus for a molding machine, the movable platen is supported by the tie bars via the elongated guide bushes. Therefore, when the movable platen is moved forward/backward by a driving device upon mold opening/closing, supporting portions are sometimes distorted to disable smooth movement For this reason, a large driving force is required. Furthermore, if a high molten metal pressure is generated upon injection of a molten metal or the like into metal molds to slightly distort the movable platen via the metal molds, a stress of the distortion abrades or damages the guide bushes or the tie bars.